Devices having bonded conductive and insulating substrates are fabricated with many different materials. Some of these devices require a sealed chamber with an external electrical contact to an internal electrically conductive element. Devices that typically have a requirement for this type of structure include capacitive pressure sensors. Considering the silicon variety of capacitive pressure sensors, one feature some have in common is a pressurized chamber for storing a reference pressure. This chamber is typically formed during the fabrication process and includes a passageway to the external surface of the chamber. This passageway is used to extend an electrical contact from the inside of the chamber, where an electrical element is located, to the outside of the chamber where it can be connected to an external measurement circuit. During manufacture the passageway to the chamber must be sealed after a reference pressure is provided in the chamber. This passageway must be both electrically conductive and sealed, to capture the aforementioned reference pressure. A combination of metal and glass materials are often used to create these sealed chambers and attendant passageways. These materials are fused into a structure by many different processes. These processes can include the disposing of a liquefied metal into a glass structure having a passageway therein, and then the solidification of the metal forming a fused interface sealing the chamber.
All of these metal and glass structures can suffer from a common problem. This problem is the failure of the fused interface, thereby leaking the stored reference pressure from the chamber. This happens when the metal and glass structures are subjected to a large temperature transition, or many large temperature transitions. These failures occur because of the difference in the thermal coefficient of expansion of metal and glass. For example, if the metal is solder, the solder will shrink to a much greater extent than the glass during a falling temperature transition. This results in a large strain at the fused interface, causing a crack in the glass, a crack directly between the solder and the glass, or a crack in the solder. In some cases many thermal transitions may be necessary before these failures occur.
In FIG. 1 a specific example is illustrated. This figure shows a cross section of a device having a sealed chamber with an external electrical contact to an internal electrically conductive element, in this case a prior art capacitive pressure sensor. This sensor uses a metal and glass structure to seal a first passageway 107 between a pressurized sealed chamber 125, in which a reference pressure is stored, and a first surface 103 of a glass substrate 101, where electrical interconnection to the capacitive element is provided. The glass substrate 101 with the first surface 103 has an opposing second surface 105. The first passageway 107, and a second passageway 109 are provided through the glass substrate 101. The first passageway 107 is then processed to include a metal layer 111 that extends to a first predetermined area 113 on the opposing second surface 105. The second passageway 109 is also processed to include a metal layer 115 that extends to a second predetermined area 117 on the opposing second surface 105 of the glass substrate 101.
A semiconductor substrate 119 is then attached to the opposing second surface 105 of the glass substrate 101 at locations shown by reference number 121 and to the second predetermined area 117. This attachment between the glass on the opposing second surface 105 and the semiconductor substrate 119, at locations shown by reference number 121, is performed by anodic, or electrostatic, bonding. This anodic bond seals the glass and semiconductor substrate at locations shown by reference number 121. The bond between the metal layer 115 that extends to the second predetermined area 117 and the semiconductor substrate 119, indicated by reference number 129, is the result of a chemical reaction that fuses the semiconductor substrate 119 and the metal layer 115 with heat and pressure.
The chamber 125 is pressurized and then a quantity of solder 123, 127 is melted into the first and second passageways 107, 109. The solder 123 in the first passageway, when cool, forms a interface for sealing the chamber 125 and provides an electrical contact to an element of a capacitor, formed by the metal layer 111 covering the first predetermined area 113. The solder 127 in the second passageway 109 provides an electrical connection to a second element of the capacitor formed by the semiconductor substrate 119. The solder 123, 127 formed in the first and second passageways 107, 109 is used to connect the capacitor to an external measurement circuit. When this capacitor is subjected to differing pressures, a portion of the semiconductor substrate 119 is displaced, in relationship to the metalized first predetermined area 113, causing a change in distance between the elements of the capacitor and thus capacitance.
The construction and sealing of the first passageway 107 is of particular concern. The temperature coefficient of solder and glass is substantially different. When the solder and glass structures are subjected to a temperature transition they expand or contract at different rates because of differing thermal coefficients of expansion. The differing rates cause stress to build up in the solder 123, particularly at the location denoted by reference number 102, and the glass substrate 101. This is undesirable because during large temperature transitions, at the location denoted by reference number 102, the glass substrate 101 and solder 123 will be over stressed and either or both will crack. This cracking causes the seal to be broken. When this seal is broken the reference pressure stored in chamber 125 is released and the sensor no longer can function as designed. Similar cracks in the glass-solder interface for the second passageway 109 have no effect on the chamber 125 because of the anodic bond at the locations shown by reference number 121. This anodic bond isolates the chamber 125 from any breaches in the second passageway 109.
What is needed is an improved device having a sealed chamber with an external electrical contact to an internal electrically conductive element.